1. Field of the Invention:
This invention relates to a method of and apparatus for coating non-conductive substrates, e.g. glass, with transparent coatings having uniform optical properties.
2. Description of the Prior Art:
In general, transparent substrates are coated with a transparent metal film for use as transparent windows or outside walls in a building such as a skyscraper or other multi-story structure. These substrates may be especially advantageously employed as one of the plates which make up multiple glazed units. It will be appreciated that uniformity of coating in such cases is especially important because otherwise the reflected color portions of the building differ sharply from that of other portions, thus distracting from its appearance.
In general, the prior art teaches a process whereby glass plates are conveyed along an article movement path through a sequence of rinse sprays and coating sprays. After a plate is rinsed, normally with water, it is covered with a layer of water. When the plate enters the coating spray, this layer of water is normally pushed toward the trailing edge of the plate and tends to accumulate toward the trailing edge.
As more water accumulates at the trailing edge, the water tends to flow back into the coating spray. The problem is especially severe at the trailing edge where the water is retained by the edge of the plate. After reaching equilibrium thickness at the trailing edge, some of the water flows back into the coating spray so that the glass plate is sprayed while it is covered with one or more puddles of water of non-uniform thickness. As a result, there is an uneven dilution of the coating solution, causing the transparent coating on the plate to vary in thickness, in absorption and in reflectance. This is extremely pronounced adjacent to the trailing edge of the plate.
Other problems associated with the prior art process of coating a piece of glass are (1) finger-like fronts of sprayed-on solution form between each rinse spray and coating spray which manifests itself in streaks in the direction of travel of the plate; (2) splashing of the rinse medium in the coating area causes streaks and mottle in the coating and; (3) the spray nozzles disturb the rinse film in the spray coating zone making the rinse film non-uniform which causes the coating to vary in thickness, in absorption and in reflectance.
In U.S. Pat. Application Ser. No. 159,747, filed on July 6, 1971, in the name of the present inventor and entitled "Squeegee/Shield" and U.S. Pat. application Ser. No. 324,730, filed on Jan. 18, 1973, which is a division of U.S. Pat. application Ser. No. 159,747 now U.S. Pat. Nos. 3,761,305 and 3,782,328 respectively, there is disclosed a solution to the problem caused by the accumulation of water at the trailing edge and the finger-like fronts. In the above-mentioned applications, there is suggested the use of a squeegee/shield combination ahead of the coating spray to insure that there are no puddles or finger-like fronts of rinse water at the coating spray. The squeegee is made of soft rubber or plastic and it prevents the formation of finger-like fronts and also protects the glass surface from splashing and defects associated with splashing.
However, the squeegee/shield combination has limitations. For example, there is always a chance of damaging the coated layers on the surface when solid particles become trapped between the glass and the squeegee or in the squeegee material itself abrading the surface layers.
In U.S. Pat. No. 3,793,054 there is disclosed another solution to the problem of accumulation of water at the trailing edge and the finger-like fronts as well as eliminating the problem of the particles abrading the surface. A reciprocating crossfire set of rinsing facilities rinses the surface of the glass sheet while maintaining a uniform thickness of rinsing medium in an area on the glass surface where a subsequent spray coating operation is to be carried out. The rinsing facilities are arranged such that the rinsing medium spray is angled obliquely downward, rearward and outward and the rinsing medium is supplied at a rate sufficient to force excess rinsing medium off the side and the trailing edge of the sheet rather than being delivered into the spray coating zone or accumulating along the trailing edge portion of the plate. The finger-like puddles are eliminated by moving the coating spray and the crossfire set of rinsing facilities in unison and positioning the coating spray and rinsing spray relative to one another such that the rinse spray and coating spray intercept about 1/8 to 1/4 inch at the surface of the glass.
Although the angled crossfire rinse eliminates the problems of streaking and puddling in addition to abrading, there are limitations. More particularly, during reciprocation, a zigzag spray pattern results from the transverse movement of the coating spray and the forward movement of the conveyor. Setting practical speed limits on both variables plus the interaction between the transversing rinsing and coating sprays has been found to be critical for uniform coatings. Further, experience has taught that fast-moving transverse bridges can create serious wear and tear, as well as maintenance and contamination problems. In addition, in order to eliminate puddles in the coating zone, the rinsing spray intercepts about 1/8 to 1/4 of the coating spray. To maintain this arrangement requires considerable care in arranging the angled crossfire rinsing sprays and the coating sprays.